fix a string bug that occurs when using both dshell and Json module

fix a string bug that occurs when using both dshell and Json module.
if over 16 bytes of string is passed to json value, it becomes incorrect string. 

* input

```
import("dshell");
import("konoha.json");

void test() {
	Json j = new Json();
	j.setString("16bytes", "0123456789012345");
	System.p(j.getString("16bytes"));
	j.setString("17bytes", "01234567890123456");
	System.p(j.getString("17bytes"));
}

test();
```

* output

```
 - (js.k:7) 0123456789012345
 - (js.k:9) 01234567890123456r/sbin:/sbin
```